Paraxylene is commercially important because it is a precursor for polyester fibers, films, bottle plastic, and the like. A typical equilibrium mixture of xylene isomers is contains only about 22-24 wt % paraxylene. In a conventional aromatics complex, paraxylene is separated from its C8 aromatic isomers by a process including adsorptive separation or crystallization processes, to produce a paraxylene-enriched stream and a paraxylene-depleted stream (“raffinate”). The raffinate is isomerized to equilibrium and recycled for paraxylene separation.
There are a number of known commercial processes to accomplish the isomerization of raffinate to equilibrium. For instance, there is a process that takes the paraxylene-depleted xylenes stream, which also includes ethylbenzene, and de-alkylates ethylbenzene to produce benzene and ethylene, while isomerizing the xylenes to an equilibrium mixed xylene product. The ethylbenzene dealkylation and xylene isomerization reactions can be carried out in a single step or the steps can be decoupled and accomplished step-wise in a dual-bed catalyst system. For illustration of the latter process, see, for instance, U.S. Pat. Nos. 5,004,855; 5,516,956; and 6,028,238.
Rhenium (Re) is used to give metal functionality to certain zeolite-based high activity isomerization catalysts, such as in one or more of the aforementioned prior art. Re gives high olefin saturation activity with minimal aromatic ring saturation in the ethylbenzene dealkylation reaction in the presence of hydrogen. However, it has been discovered that during conventional start-up procedures in a xylenes isomerization process, the rhenium metal-modified catalyst readily catalyzes the ammonia synthesis reaction when nitrogen and hydrogen are present together and in contact with said catalyst, even at at relatively low temperature and pressure, as shown below:N2+3H2→2NH3 
Ammonia formation occurs over the rhenium promoted xylene isomerization catalyst during the unit start-up, which includes introduction of nitrogen gas to remove oxygen and moisture, followed by the introduction of hydrogen for the purpose inter alia of reducing the metal, while at the same time increasing the temperature of the system to operational temperatures. Thus, the start-up procedure, including catalyst dry-out phase and introduction of hydrogen usually results in a relatively long period with significant nitrogen concentration in hydrogen gas at high temperature and pressure, typically above 180° C. and a pressure above 0.5 MPa. These gases are recycled in typical isomerization systems. In several commercial applications using this catalyst, ammonia was detected in the recycle gas during the catalyst dry-out phase. While ammonia production from hydrogen and nitrogen is per se is generally highly desirable, in the context of zeolite-based xylene isomerization it is not, as ammonia is a poison for the acid sites of the zeolite-based catalyst. Extended exposure results in potentially severe loss of catalyst activity. Since the rhenium promoter inherently provides high activity for ammonia synthesis, it is desirable to minimize the exposure of the rhenium-promoted zeolite xylene isomerization catalyst to ammonia in order to maintain its high performance characteristics.
The present inventors have surprisingly discovered a procedure that decreases the amount of ammonia produced during the catalyst dry-out phase and introduction of hydrogen and minimizes catalyst poisoning by ammonia.